Spontaneous Degrafting of Weak and Strong Polycationic Brushes in

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Spontaneous Degrafting of Weak and Strong Polycationic Brushes in Aqueous Buffer Solutions Yeongun Ko† and Jan Genzer*,†,‡ †

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Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, United States ‡ Global Station for Soft Matter, Global Institution for Collaborative Research and Education (GI-CoRE), Hokkaido University, Sapporo 060-0808, Japan S Supporting Information *

ABSTRACT: Polymers grafted to substrates have traditionally been considered stable because of the covalent bonds that hold the polymers attached to the substrate. However, several recent reports have indicated that grafted polymers may detach from substrates under specific conditions. In this work, we report on a systematic study of polymer degrafting involving polycationic brushes with different degrees of quaternization (DQ, mol %), which have been incubated in three different buffer solutions (pH 4, 7.4, and 9) with the same ionic strength of 0.05 M. We have varied the molecular weight (MW) and grafting density (σ) of the polymer brushes using a combinatorial setup to examine the effect of MW, σ, and DQ on polymer degrafting. Furthermore, we explored the effect of the bonding environment at the base of the initiator (mono- vs. tri-functional) of the grafted polymer layer at the substrate on the overall stability of polymer brushes on the substrate. The two major findings in this paper are (1) degrafting of polycationic grafts from flat silica substrate increases with increasing DQ of the polymer and (2) polymer degrafting likely occurs both in the initiator ester group and the silane head-group at the silicon substrate.

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INTRODUCTION

Several researchers reported on degrafting of hydrophilic polymer brushes in aqueous solution20,21 and even degrafting of hydrophobic polymers in organic solvents.22 Polymer brush degrafting involves a breakage of a covalent bond in the initiator linker with the assistance of a mechanical force due to swelling of the brush.23 Many researchers have attempted to comprehend breaking a covalent bond in grafted polymers to disclose the mechanism of chain breaking by considering click chemistry,24 photo-cleaving,25−27 sonication-induced chain scission,28,29 strong acid or base,30−33 and so forth.34−37 In our own previous work, we considered degrafting of polymer brushes from silicon substrates using tetrabutylammonium fluoride (TBAF). The fluorine ions in TBAF selectively cleave the Si−O bonds, leading to degrafting.38−41 We classified this mode as “on-demand degrafting”. In a series of studies, we reported that polyanionic brushes detached from substrates upon increasing pH of the solutions that induced strong swelling due to electrostatic charging and concurrent hydrolysis of ester bonds in the initiator.42−45 This is considered as “spontaneous degrafting”. Here, we explore the effects of charge density, MW, and grafting density on degrafting of weakly and strongly charged polycationic grafts

Polymer brushes feature polymer chains which are physically or chemically attached to a surface.1−3 Here, we consider chemically grafted polymer chains anchored to a flat impenetrable substrate. The grafted polymers exhibit high stability and chain expansion in good solvents because every polymer chain is covalently anchored to the substrate. At low grafting density of polymer grafts, the spacing between grafted points on the substrate is larger than the radius of gyration (Rg) of the grafted polymer.4 In this state (called “mushroom regime”), the polymer grafts do not interact with one another. At high grafting densities, the distance between grafted points is much smaller than Rg (called “brush regime”) and the polymers are sterically hindered by neighboring chains and swell in the direction normal to the substrate when incubated in good solvents. Polymer brushes with high grafting densities are typically prepared by the “grafting from” approach, which involves anchoring initiator molecules onto a solid substrate, followed by surface-initiated polymerization.1,3 Polymer brushes have been studied theoretically and experimentally for decades to understand and tune the properties of surfaces, including anti-fouling, low-friction, responsive functions, and so forth.5−13 Numerous researchers have focused on studying the swelling behavior of polymer brushes as a function of the brush molecular weight (MW) or grafting density (σ) in good solvents.14−19 © XXXX American Chemical Society

Received: June 30, 2019 Revised: July 28, 2019

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DOI: 10.1021/acs.macromol.9b01362 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

HCl and NaOH. The ionic strength was maintained at 0.05 M with NaCl salts. Characterization. We employed variable angle spectroscopic ellipsometry (VASE) (J.A. Woollam Co.) to measure the thickness and refractive index of polymer thin films. The ellipsometry data were analyzed using WVASE32 software (J.A. Woollam Co.). Every measurement was performed at two angles of incidence (60° and 65° relative to the normal) between 400 and 800 nm. A hot stage (FP82HT, Mettler Toledo) connected to a central processor (FP90, Mettler Toledo) was attached to the VASE sample stage. This assembly enabled controlling the temperature of polymer films during the ellipsometry measurements. Relative humidity was not controlled during the experiments; it was measured to be ∼50% at room temperature. We measured the thickness of PDMAEMA and qPDMAEMA at elevated temperatures (100 °C) to minimize water sorption.53 We used a single layer model to fit the ellipsometric data. This model comprises Si substrate, SiOx layer (thickness 1.5 nm), and a Cauchy layer (n = An + Bn/λ2, where n is refractive index and An and Bn are fitting parameters). The DQ (mol %) was estimated by using the refractive index at 600 nm. In our previous work, we demonstrated that DQ obtained from elemental analysis is linearly proportional to the refractive index which can be obtained from VASE measurements.53 The refractive index of pure PDMAEMA was measured and that of qPDMAEMADQ100 was estimated by extrapolation. These values were used in the effective medium approximation (EMA). DQ (mol %) could be obtained from DQ (vol %) by using density and MW of the polymers. We have used the following equation to relate DQ to n: DQ = (835.10 × n − 1233.77). To measure the wet thickness of polymer brushes, we employed a liquid cell featuring fixed windows fixed at 70° (relative to the vertical direction) for the incoming and outgoing light signals of the ellipsometer. All wet thicknesses were measured after 10 min of incubation for each specimen in the liquid medium. We used a twolayer model to fit the ellipsometry data. This model comprises Si/ SiOx/Cauchy1/EMA(Cauchy2 + water). The EMA uses a volumeaveraged layer combining the optical properties of the polymer and water. The Cauchy1 and Cauchy2 use fixed An and Bn values as 1.4686 and 0.005, respectively. The thicknesses of Cauchy1 and EMA layer and the volume fraction of water in the EMA were used as the fitting parameters. The swelling ratio (H/h) was obtained from wet thickness divided by dry thickness measured at 100 °C. From our previous work, the difference in the “true” dry thickness (i.e., thickness at ambient conditions) for a polymer film having ∼100 nm thickness 0, the extent of degrafting increases slightly with increasing N and DQ. We tentatively explain this behavior by considering the effect of dispersity in MW (D̵ ) of the brushes. There are only a few studies discussing the effect of D̵ on the conformation of polymer brushes, in which the authors have shown that the density profiles of the polymers changes from a parabolic concave to a linear to a convex shape when D̵ of the brushes

initiators are susceptible to mechanical forces due to chain solvation and stretching. The hydroxide ions in pH 9 can catalyze the hydrolysis at the initiators, resulting in severe degrafting. Although the brushes are heavily charged at pH 4, the degrafting is lower than that in polymer brushes incubated in pH 9. We observe the same trend with PMETAC (qPDMAEMA-DQ100) brushes. The exposure of PMETAC brushes (albeit with a different counterion) to solutions of pH 9 results in the highest degrafting among all cases studied. Assuming that PMETAC has a comparable σ and N as PDMAEMA and qPDAEMA-DQx samples, the PMETAC’s tendency to degraft the most among all samples studied, supports the notion of mechanical force-induced degrafting mechanism at the initiator site due to strong chain swelling. It is interesting to compare the results of PDMAEMA, qPDMAEMA-DQ79, and PMETAC brushes incubated in pH 4. The PMETAC and qPDMAEMA-DQ79 brushes degraft more strongly than PDMAEMA brushes even though they have nearly the same charge density. The degrafting of PMETAC brushes in pH 4 is nearly identical to that of qPDMAEMA-DQ79. We attribute this behavior to the steric hindrance effect originating from the extra methyl group attached to the quaternary ammonium as well as the presence of the “bulky” counterions. This observation should be confirmed by additional experiments involving longer alkyl chain substituents (e.g., ethyl iodide or propyl iodide). From the result displayed in Figure 1, the mechanical force originating from brush swelling applied at the initiator site is a critical parameter that governs degrafting of polymers from the substrate. The mechanical force can be tuned by changing DQ, as demonstrated by the data plotted in Figure 2. All qPDMAEMA brushes (except perhaps for PMETAC brushes) possess the same average grafting density and degree of polymerization; the only difference among the samples is their DQ. The extent of degrafting and degrafting kinetics increases with increasing DQ. From eq S5, the elastic force acting on each graft increases nonlinearly with increasing wet thickness or increasing DQ. By plotting the normalized thickness as a D


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Figure 4. (a) Normalized dry thickness of qPDMAEMA-DQ82 brushes having different grafting densities as a function of the incubation time in pH 9 solution. The legend indicates the initial dry thickness (h0) of qPDMAEMA-DQ82 before incubation. All data were normalized by 132.0 nm (hN), which is the highest thickness of polymer brushes with the highest initial grafting density. The lines are meant to guide the eye. (b) The same data is represented as a function of the initial normalized dry thickness of qPDMAEMA-DQ82 brushes. All data are normalized by initial dry thickness (i.e., from right to left data normalized by 132.0, 112.6, 94.2, 72.5, and 55.1 nm).

changes from monodisperse to moderately disperse (D̵ ≈ 1.15) to highly disperse (D̵ ≈ 2), respectively.58−60 The density profile is derived from brush conformations, which, in turn, affects the elastic force acting on each chain. Thus, the elastic force may vary in brushes with different dispersity. For the surface-initiated ATRP, the longer chains may possess higher dispersity in MW.61,62 We also prepared specimens featuring grafting density gradients of PDMAEMA and qPDMAEMA brushes. After forming a uniform self-assembled monolayer (SAM) of silanebased initiators on a silicon substrate, the sample was immersed vertically in a stepwise fashion into a solution featuring 0.1 M TBAF in DMA at 50 °C.40 After that, the substrate was placed in a vial filled with the ATRP solution to achieve a grafting density gradient of polymer brushes (see Supporting Information). This approach may lead to small variations in MW of the brushes due to the different confinement at different grafting densities.62 From our past experience, this effect, if present, is negligible. However, because the extent of degrafting is nearly constant regardless of the MW variation as shown in Figure 3, we can assume here that the MW effect on degrafting is negligible (vide supra). In Figure 4a, we plot the dry brush data normalized by 132 nm, which is the initial dry thickness of the sample in the highest grafting density region (h0). The data in Figure 4a reveal that the extent of degrafting increases with increasing incubation time in the pH 9 solution. The relative extent of degrafting is higher in samples with higher initial grafting density. This is not surprising because densely grafted brushes are stretched away from the substrate due to excluded volume interactions with their neighbors and thus experience higher elastic force.33 The number of degrafted chains from the substrate decreases with decreasing polymer grafting density. By normalizing the degrafting data by the corresponding initial dry thicknesses, we gain insight into the grafting density effect on degrafting (cf. Figure 4b). The results show only a small difference between the lowest and the highest grafting densities. Further analysis will be described below in Figure 7. The degrafting kinetics of grafting density gradient can be superimposed onto a single master curve by horizontally shifting the data in Figure 4a (cf. Figure 5). The rational is that regardless of the initial grafting density, the degrafting process should follow the same time dependence. In our analysis, we omitted the first degrafting point. In our previous paper, we

Figure 5. Normalized dry thickness of qPDMAEMA-DQ82 brushes having different grafting densities (from Figure 4a) as a function of the shifted degrafting time. The inset displays a linear relationship between shift time and initial normalized thickness (hN = 132 nm).

attributed the initial degrafting data to removing multilayers/ imperfectly bound chains present at the very surface that would be liberated first from the grafted polymer layer.41 Employing the “data-shifting” procedure brings all experimental data to a master curve. The inset in Figure 5 displays a linear dependence of the employed time shift as a function of the initial normalized grafting density of polymer brushes (h0/ hN). Up until now, we used tBMPUS as the initiator. This molecule features a tri-functional silane head-group that enables the attachment to the substrate or/and neighboring silane molecules by employing three covalent bonds. To study the effect of the silane head-group structure on the stability of brushes, we prepared SAMs featuring mBMPUS as an initiator. mBMPUS features one functional unit and two methyl groups attached to the silicone head-group. This initiator should form a true monolayer on the supporting substrate. We grew PDMAEMA brushes from both mBMPUS and tBMPUS in a vial filled with ATRP solution and quaternized them to obtain qPDMAEMA-DQ82 brushes. The mono-functional silane has a lower grafting density than the tri-functional silane because the mono-functional silane experiences steric hindrance from two methyl units in the silicone head-group.63,64 Unfortunately, we could not estimate either the degree of polymerization or the grafting density. However, we know that the effect of the degree of polymerization on degrafting is negligible for PDMAEMA and relatively small for qPDMAEMA (cf. Figure 3b). Figure 6 shows the results of degrafting E


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involves base-catalyzed hydrolysis that takes place in the initiator centers. The BMPUS initiators have two kinds of bonds that can be hydrolyzed: (1) ester/amide bond close to the initiating center, and (2) siloxane bond. Galvin and coworkers67 studied the effect of ester vs. amide in the BMPUS initiator on the stability of polymer brushes and concluded that amide bonds are less prone to hydrolysis relative to the ester bonds. In both cases, they use a BMPUS molecule that had a tri-functional silicone head-group. In this work, we only used ester-based initiators but varied the number of bonding units in the silicone head-group. We observed that siloxane bonds are susceptible to hydrolysis, which is likely the main mechanism that leads to the degrafting in the current system. We state so because if the hydrolysis reaction occurred primarily in the ester bond, the extent of degrafting would have been comparable for brushes grown from both mBMPUS and tBMPUS initiators because both initiator molecules possess one ester bond each. We note, however, that hydrolysis in the ester bond can still take place.66,67

Figure 6. Normalized dry thickness of PDMAEMA (open) and qPDMAEMA-DQ82 (solid) brushes having different silane-based initiator structures with incubation time. The lines are meant to guide the eye. The initial dry thickness (h0) of mBMPUS-based brushes were 47.7 and 60.2 nm for DQ0 and DQ82, respectively. The h0 values of tBMPUS-based brushes were 93.4 and 119.8 nm for DQ0 and DQ82, respectively. The cartoons on the right describe a tentative structure of the tBMPUS (black border) and mBMPUS (violet border) initiators on the surface.

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DISCUSSION

We employed the stretched exponential function, a continuous sum of exponential decays, to fit our experimental data.68 We assume a pseudo first-order reaction kinetics with the apparent rate constant k(t),

PDMAEMA and qPDMAEMA-DQ82 from both mBMPUSand tBMPUS-grown initiator centers. All data demonstrate that polymer brushes grown from mBMPUS experience larger extent of degrafting than those grown from tBMPUS. This result is very important as it shows that the silane head-group structure plays a crucial role in controlling the stability of polymers grafted to the substrate. We can make a few conclusions from the experimental results discussed previously and from earlier publications.40,44,45,65−67 From the data in Figure 1, degrafting

k(t ) = k 0·t β− 1

(2)

dσ = −k(t ) ·σ dt

(3)

i k y σ = σ0·expjjjj− 0 ·t β zzzz k β {

(4)

Figure 7. Normalized dry thickness of qPDMAEMA brushes at 100 °C having (a) different DQ (from Figure 2a) and (b) different grafting density for qPDMAEMA-DQ82 (from Figure 4a). The closed symbol depicts experimental data and the corresponding colored lines are the best fits to eq 6. (c,d) Fitting parameters for (a) and (b). The values of k0 correspond to the right ordinate in plots (c) and (d). The corresponding colored lines are splines and represent the guide to the eye. F


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where k0, t, β, and σ0 are intrinsic (or initial) rate constant (min−1), incubation time (min), a stretching exponent, and the grafting density (chains/nm2), respectively, before incubation in pH 9 solution. While for β = 1, the rate constant is timeindependent; for 0 < β < 1, the exponential function is stretched and k(t) decreases with increasing time. Considering the equilibrium state at infinite time without further degrafting, where the grafting density is σeq, Equation 4 can be rewritten as i k y σ = σeq + (σ0 − σeq) ·expjjjj− 0 ·t β zzzz k β {

CONCLUSIONS We have explored degrafting of weak and strong polycationic brushes based on PDMAEMA and qPDMAEMA, respectively, by incubating them in pH 4, 7.4, and 9 solutions at 0.05 M ionic strength with monovalent ions. The extent of degrafting in qPDMAEMA brushes increased with increasing degree of quaternization and incubation time in solution. The extent of degrafting is the highest in pH 9, supporting the notion that degrafting involves the base-catalyzed hydrolysis reaction at the initiator site. Increasing the degree of dissociation (due to increased pH and/or increased DQ) increases brush swelling, which in turn, increases the elastic force applied on the initiators. This results in severe degrafting of the brushes from the substrates. The degrafting rate increases with increasing grafting density of polymer brushes (Figure 4a). The degrafting mechanism is the same for all polymer brush systems studied, as revealed from the data in Figures 4b, 5, and 7. It involves time-dependent variation in the elastic force acting on grafted chains that decreases with increasing time and gives rise to time-depended conformations of polymer grafts on the substrate. Interestingly, we observed that the degrafting process depended on the MW of qPDMAEMA brushes. Specifically, we observed higher degrafting with increasing MW of the polymers and DQ. While not predicted by theory, we tentatively explained the behavior by considering MW dispersity of the brushes that may affect the instantaneous conformation of polymer chains on the substrate and thus the local elastic force. By comparing the degrafting of polymer brushes from tBMPUS and mBMPUS, we concluded that the polymer brush degrafting took place primarily at the siloxane head-group of the initiator that held the chains attached to the substrate.

(5)

or can be normalized by σN (or hN, cf. Equation 1) σeq σeq yz ij σ i y σ h zz·expjjj− k 0 ·t β zzz = = + jjj 0 − z j z j σN σN hN σN σN z{ k β { k

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(6)

For brushes having different DQ values (Figure 7a), σN is equal to σ0. In the case of the grafting density gradient brushes (Figure 7b), σN represents the highest initial grafting density before TBAF degrafting (i.e., hN ≡ the highest initial dry thickness = 132.0 nm). The experimental data (closed symbols) and the best fits (lines) are plotted in Figure 7a,b, and the fitting parameters are plotted in Figure 7c,d. All data points are fitted with R2 > 0.994. In Figure 7c, all fitted σ0/σN values are equal to unity as expected. σeq/σN values decrease with increasing DQ, implying that the grafting densities at equilibrium decrease due to strong mechanical forces that increase with increasing charge density. At DQ ≥ 71, the σeq/σN values reach ∼0. Thus, high charge density can remove all the brushes from the substrate. At DQ0, β is ∼1, implying that the kinetics resembles an exponential decay. By increasing DQ, the β values approach ∼0.5, noting that the apparent rate constant decreases with increasing time. The intrinsic rate constant k0 displays a non-linear increase with DQ, supporting the results plotted in Figure 2b. In Figure 7d, σ0/σN values are well fitted to the initial normalized thicknesses (h0/hN). Because all brushes have a high charge density, the σeq/σN values reach values ∼0 at equilibrium. The β values are ∼0.6 for nearly all values of h0/hN, which further validates the result shown in Figure 4b and implies that polymer brush degrafting follows the same degrafting mechanism. This is further justified by the master plot in Figure 5. The k0 values increase slightly with increasing grafting density of the polymers. The degrafting kinetics for qPDMAEMA-DQ82 brushes grown from mBMPUS was also fitted by adopting the same analysis as above (data not shown). β was ∼0.68, indicating that the kinetics of degrafting was comparable to that of polymer brushes grown from tBMPUS. This implies that the kinetic of degrafting is independent of the structure of the head-group in the silane initiator. Considering all results, we suggest that degrafting of densely populated polycationic brushes on surfaces changes the temporal grafting density and thus the brush conformation on the substrate. This, in turn, leads to time-dependent reduction of the elastic force that acts on the grafted chains and reduces the extent of degrafting with progressing time. The whole process is characterized by a time-dependent and decreasing rate constant describing the degrafting phenomenon.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01362. Details pertaining to preparation and characterization of polymer films; dry thicknesses of PDMAEMA brushes with MW gradient and grafting density gradient; and FTIR spectra of tested polycationic brushes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yeongun Ko: 0000-0001-5770-6707 Jan Genzer: 0000-0002-1633-238X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Science Foundation, Grant no. DMR-1404639. REFERENCES

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